Solar collector

ABSTRACT

A solar collector includes a first wall and a second wall facing one another, which between them delimit a housing, the first wall being transparent and intended to face in the direction of the solar radiation incident upon the collector, and an absorption device for absorbing the energy derived from the solar radiation and positioned in the housing, the absorption device including at least one pipe through which a heat-transfer fluid flows. In addition, the collector includes at least one transparent spacer arranged between the first wall and the absorption device in the region of the pipe.

The present invention relates to a flat solar collector intended to be mounted on a structure, particularly a roof or a façade of a building.

A solar collector is a module designed to convert the energy originating from solar radiation into thermal energy collected in a heat-transfer fluid. Conventionally, a flat solar collector comprises two walls facing one another and which between them delimit a housing accommodating energy conversion elements, generally in the form of an absorber panel thermally connected to one or more pipes through which the heat-transfer fluid flows. At least one of the two facing walls is transparent and intended to face in the direction of the solar radiation incident upon the collector so as to permit good transmission of solar radiation towards the energy conversion elements.

In order to increase the energy conversion efficiency of such a solar collector, it is known practice to create a vacuum in the housing accommodating the energy conversion elements, which makes it possible to limit thermal losses through convection and molecular conduction. In such instances, in order to counter the compressive force applied to the walls of the collector as a result of the external atmospheric pressure, the collector is fitted with spacers that make it possible to maintain a constant distance between the facing walls. The energy conversion elements are also advantageously kept some distance away from the walls that delimit their accommodating housing, again with a view to limiting thermal losses through contact at these walls.

WO-A-87/06328 describes an evacuated solar collector structure in which the spacers are in the form of rods, which rest between the two facing walls of the collector. These rods are distributed in the collector some distance away from the pipes in which the heat-transfer fluid flows and pass through the absorber panel. When the rods that form the spacers are made of metal, thermal losses are likely to occur at these rods. In addition, because the absorber panel is pierced with holes through which these rods can pass, the active area of the panel available for absorbing the energy originating from the solar radiation is reduced, and this limits the energy conversion efficiency of the collector. Another disadvantage with this known solar collector structure is that the rods that form the spacers have to be fitted into the collector individually and this increases the time and cost involved in manufacturing the collector. Further, this known collector is of great thickness, which means that it cannot be incorporated esthetically into a roof or façade of a building.

It is these disadvantages that the invention more specifically sets out to remedy by proposing a solar collector having a structure that is optimized both in terms of thermal distribution in the collector and in terms of mechanical strength of the collector, making it possible to improve the energy conversion efficiency of the collector, this solar collector also having a minimized size and a simple method of manufacture.

To this end, one subject of the invention is a solar collector comprising:

-   a first wall and a second wall facing one another, which between     them delimit a housing, the first wall being transparent and     intended to face in the direction of the solar radiation incident     upon the collector, and -   absorption means for absorbing the energy originating from the solar     radiation and positioned in the housing, these absorption means     comprising at least one pipe through which a heat-transfer fluid     flows,

characterized in that it further comprises at least one transparent spacer arranged between the first wall and the absorption means in the region of the pipe.

Within the meaning of the invention, a transparent element is an element that is transparent at least in the wavelength ranges of the solar radiation that are of use for the convertion by the absorption means of the energy originating from the solar radiation into thermal energy. In addition, a spacer is said to be positioned between the first wall and the absorption means at or in the region of a pipe when it is positioned between the first wall and the pipe, with or without the interposition of other absorbing elements between the spacer and the pipe. The spacer is then in thermal contact with a relatively cooler part of the absorption means, because the heat-transfer fluid flows through the pipe.

Thanks to the fact that each spacer is positioned in the region of a heat-transfer fluid circulation pipe, that is to say at a cold point of the absorption means, the thermal losses across the spacer are limited. In addition, the transparency of the first wall and of each spacer positioned between the first wall and the absorption means guarantees good transmission of solar radiation towards the absorption means of the collector. The collection by the absorption means of energy originating from solar radiation and the energy conversion efficiency of the collector are thus optimized.

According to other advantageous features of a solar collector according to the invention, considered in isolation or in any technically feasible combination(s):

-   the absorption means comprise an absorbing panel thermally connected     to the pipe, the spacer being arranged between the first wall and a     portion of the absorber panel in thermal contact with the pipe; -   the spacer has a shape that is rounded in the direction of the     absorption means; -   the spacer is made of glass; -   the spacer is of one piece with the first wall; -   the first wall and the spacer are made of toughened glass, the     spacer being heat toughened with the first wall; -   the first wall is made of glass, the spacer being a feature in     relief with respect to the first wall obtained by rolling the first     wall; -   the spacer is a spacer made of chemically toughened glass; -   the spacer has a thickness less than 4 millimeters, preferably less     than 2 millimeters when it is a chemically toughened spacer, and     preferably less than 1 millimeter when it is a spacer of one piece     with the first wall; -   the collector comprises at least one pair of spacers arranged one on     each side of the pipe, comprising the first spacer between the first     wall and the absorption means and a second spacer between the second     wall and the absorption means; -   the first and second spacers of each pair of spacers are positioned     in alignment with one another in a direction transverse to the     midplanes of the first and second walls; -   the second spacer is of one piece with the second wall; -   the second wall is made of metal, and the second spacer is a feature     in relief with respect to the second wall obtained by embossing the     second wall; -   the second wall is identical to the first wall and symmetric with     the latter with respect to a median plane of the collector; -   the second spacer is a spacer made of chemically toughened glass; -   the second spacer has a thickness less than millimeters, preferably     less than 2 millimeters; -   the housing is fluidtight and designed to be placed under vacuum; -   the collector comprises a spacer frame between the first and second     walls; -   the collector has a thickness less than 30 millimeters, preferably     less than 25 millimeters; -   a free edge of the first wall protrudes beyond the corresponding     free edge of the second wall so that the first wall of the solar     collector is able to cover partially the first wall of an analogous     second solar collector juxtaposed with the first solar collector.

Another subject of the invention is a cladding assembly for a structure, particularly a roof or a façade of a building, comprising at least one solar collector as described hereinabove.

The features and advantages of the invention will become apparent from the following description of four embodiments of a solar collector according to the invention which is given solely by way of example and with reference to the attached drawings in which:

FIG. 1 is a cross section of a solar collector according to a first embodiment of the invention;

FIG. 2 is a section similar to FIG. 1 for a solar collector according to a second embodiment of the invention;

FIG. 3 is a section similar to FIG. 1 for a solar collector according to a third embodiment of the invention;

FIG. 4 is a schematic plan view from above of two solar collectors according to the invention, mounted on a structure, for example a roof or a façade of a building; and

FIG. 5 is a side view of a roof structure on which solar collectors according to a fourth embodiment of the invention are mounted.

For the sake of clarity of the drawing, the various elements in FIGS. 1 to 5 have not been drawn strictly to scale.

The solar collector 1 of the first embodiment, depicted in FIG. 1, comprises a transparent upper wall 2 intended to face in the direction of the solar radiation incident upon the collector, and a lower wall 4 which is likewise transparent. The walls 2 and 4 are formed of two identical sheets of toughened glass, rectangular in shape and of the order of 4 to 6 millimeters thick. The walls 2 and 4 are joined together by means of a metal frame 5 such that together and with the frame 5 they delimit a housing 3 to accommodate the absorption means of the collector 1. When the collector 1 is in the assembled configuration, the midplanes π and π′ of the walls 2 and 4 are substantially mutually parallel. A thickness direction of the collector 1, which is substantially perpendicular to the midplanes π and π′, is denoted Z.

Each of the walls 2 and 4 is fixed to the frame 5 by means of a fluidtight seal 10, particularly a gas tight seal. For preference, the seal 10 between the metal frame 5 and each glass wall 2 or 4 is obtained by soldering, using a soldering alloy, between the frame and a metallic frit deposited on the glass wall. This metallic frit, comprising a glass frit and metallic particles, is advantageously applied by screen printing to the periphery of that face of the glass wall 2 or 4 that is intended to come into contact with the frame 5, and is then baked during the heat toughening of the glass wall 2 or 4. The seal 10 obtained between the glass and the metal has good mechanical strength and sealing properties over the long term. In particular, the seal 10 allows the housing 3 in which the absorption means of the collector are located to be placed under and kept under vacuum.

These absorption means comprise a metallic panel 6, also known as the absorber panel, and a pipe 7 through which a heat-transfer fluid flows. The heat-transfer fluid is, for example, water, possibly mixed with an antifreezing agent. The absorber panel 6 is positioned between the upper wall 2 and the pipe 7 through which the heat-transfer fluid flows, so that it is able to store heat originating from the solar radiation passing through the wall 2, this heat then being transferred from the absorber panel 6 to the fluid flowing through the pipe 7. For this purpose, the pipe 7 is in thermal contact with the lower face 6A of the absorber panel 6 facing toward the lower wall 4. More specifically, the pipe 7 is positioned against the lower face 6A of the absorber panel 6 in the form of a serpentine coil, so as to maximize the area for thermal contact between the pipe and the absorber panel. As shown in FIG. 1, the absorption means 6 and 7 are preferably held at a median plane P of the collector, at some distance from the walls 2 and 4, so as to limit thermal losses through contact at these walls. The absorber panel 6, particularly made of copper or of aluminum, has a thickness in the Z direction of the order of 0.1 to 1 millimeter. The pipe 7, particularly made of copper, is of circular cross section with a diameter of the order of 8 mm.

The pipe 7 opens to the outside of the collector 1 at two inlet and outlet connectors 11 for the heat-transfer fluid, these being depicted schematically in FIG. 4. For preference, the two connectors 11 are provided on one and the same edge face of the collector 1, that is say are installed on one and the same side 51 of the rectangular shaped frame 5, each being positioned near one end 51A or 51B of the side 51. When the collector 1 is mounted on a structure 12 that is inclined with respect to the horizontal, such as a roof or a façade, the side 51 of the frame 5 that comprises the connectors 11 is preferably positioned the highest up by comparison with the other sides of the frame. It is then easy, when two collectors 1 are juxtaposed on the structure 12 to form a cladding assembly 20 as shown in FIG. 4, for the inlet connector of a first collector 1 to be connected to the outlet connector of the second collector 1 juxtaposed with it, for example using a U-shaped hose 13. A solar collector system is thus formed in which the heat-transfer fluid can flow, through a plurality of collectors in succession.

A plurality of spacers 8 and 9, in the form of pads, is provided in the collector 1 to maintain a constant distance between the upper wall 2 and the lower wall 4 when the collector is placed under vacuum. The collector 1 thus comprises a first series of spacers 8, called upper spacers, which are positioned between the upper wall 2 and the absorption means, and a second series of spacers 9, called lower spacers, which are positioned between the lower wall 4 and the absorption means. The spacers 8, 9 are distributed in the collector 1 in such a way as to form pairs of spacers. Each pair of spacers comprises an upper spacer 8 and a lower spacer 9 which are substantially aligned in the Z direction and positioned on each side of the absorption means, each time in the region of the pipe 7.

More specifically, each upper spacer 8 is positioned between the upper wall 2 and a portion 61 of the absorber panel 6 which is in thermal contact with the pipe 7, while each lower spacer 9 is positioned between the lower wall 4 and the pipe 7. The spatial arrangement of the spacers 8, 9, each time in the region of a portion of the pipe 7, allows thermal losses through the spacers from the absorption means 6 and 7 to be limited. Specifically, the pipe 7 and the portions 61 of the absorber panel 6 which are in thermal contact with the pipe 7 are cold zones in the thermal distribution of the absorption means, thus limiting the risk of thermal leaks through the spacers.

In this first embodiment, each spacer 8 or 9 is a feature in relief that protrudes with respect to the corresponding wall 2 or 4 and is formed by rolling the sheet of glass of which the wall 2 or 4 is made. In other words, the series of upper spacers 8 is a surface texture of the upper wall 2, obtained by rolling the flat surface of the sheet of glass of which the wall 2 is made, by heating the sheet of glass to a temperature at which it is possible to deform its surface using a solid object such as a metal roller which on its surface has the inverse of the texture that is to be formed. Likewise, the series of lower spacers 9 is a surface texture of the lower wall 4, obtained by rolling the flat surface of the sheet of glass of which the wall 4 is made. Advantageously, the rolled spacers 8 and 9 are toughened during the heat toughening of the walls 2 and 4. Spacers 8 and 9 are thus obtained which are incorporated into the glass wall 2 or 4 and which have good transparency and mechanical strength properties.

In the second embodiment depicted in FIG. 2, elements similar to those of the first embodiment bear identical references increased by 100. The solar collector 101 according to this second embodiment differs from the solar collector 1 of the first embodiment through the structure of its spacers 108 and 109. As in the first embodiment, the solar collector 101 comprises a transparent upper wall 102 and a lower wall 104 which is likewise transparent, these two being formed by two identical sheets of heat-toughened glass. The walls 102 and 104 between them and with a metal frame 105 to which they are attached by a fluidtight seal 110 delimit a fluidtight housing 103 for accommodating the absorption means 106 and 107 of the collector. These absorption means, which are similar to those of the first embodiment, comprise an absorber panel 106 and a pipe 107 in which a heat-transfer fluid flows. As in the first embodiment, the pipe 107 is in thermal contact with the absorber panel 106 on the side of the lower face 106A thereof.

The collector 101 comprises a plurality of upper spacers 108 and a plurality of lower spacers 109 which are intended to maintain a constant distance between the upper wall 102 and the lower wall 104 when the collector 101 is placed under vacuum. As in the first embodiment, these spacers 108 and 109 are aligned in pairs in the Z direction of thickness of the collector 101, so that each upper spacer 108 is positioned between the upper wall 102 and a portion 161 of the absorber panel 106 which is in thermal contact with the pipe 107, while each lower spacer 109 is positioned between the lower wall 104 and the pipe 107. However, in this second embodiment, the spacers 108 and 109 are not in the form of features in relief obtained by rolling but are in the form of glass beads added on to the walls 102 and 104, for example by bonding. In order to withstand the compressive force applied to the walls 102 and 104 when the vacuum is created in the housing 103, the glass beads are strengthened by chemical toughening. This chemical toughening treatment seeks, by ion exchange, to replace the alkaline ions initially present in the glass and close to the surface with other, larger, alkaline ions with a view to inducing high compressive stresses at the surface. Chemical toughening thus makes it possible to significantly increase the mechanical strength of the beads.

For this chemical toughening treatment, the glass of which the beads is made needs, prior to toughening, to contain an alkaline oxide. By way of nonlimiting example, the initial oxide may be Na₂O, it then being possible for the chemical toughening to be carried out by treatment with KNO₃, so as to replace, at least partially, the Na⁺ ions with K⁺ ions; the initial oxide may also be Li₂O, it then being possible for the chemical toughening to be carried out by treatment with NaNO₃ or KNO₃ so as to replace, at least partially, the Li⁺ ions with Na⁺ or K⁺ ions. The chemical toughening leads to an ion, notably K⁺ or Na⁺, concentration gradient perpendicular to the treated surfaces and which decreases away from these surfaces. In practice, for example in the case of a sodium/potassium exchange, the ion exchange is performed by dipping the glass beads in a bath of potassium salt raised to temperatures comprised between 400 and 500° C. The ion exchange parameters, notably the temperature and duration, are chosen to encourage a high surface stress. Ion exchange may also be assisted by an electric field.

In the third embodiment depicted in FIG. 3, the elements similar to those of the first embodiment bear identical references increased by 200. The solar collector 201 according to this third embodiment differs from the solar collector 1 of the first embodiment in that its lower wall 204 is made of metal. More specifically, the solar collector 201 comprises a transparent upper wall 202, formed of a sheet of heat-toughened glass, and a metal tray 215. The metal tray 215 comprises a bottom 204 which forms a lower wall of the collector 201, and a lateral edging 205 which extends substantially perpendicular to the bottom 204. The upper wall 202 is connected to the free edge of the edging 205 by means of a fluidtight, particularly a gas tight, seal 210. As before, the seal 210 between the metal edging 205 and the glass upper wall 202 is preferably obtained by soldering together the free edge of the edging 205 and a metallic frit applied to the upper wall 202. The walls 202 and 204 between them and with the edging 205 delimit a fluidtight housing 203 in which are housed the absorption means 206 and 207 of the collector 201, which comprise an absorber panel 206 and a pipe 207 for the flow of a heat-transfer fluid which is in thermal contact with the absorber panel 206 on the lower face 206A side thereof.

As in the previous embodiments, the collector 201 comprises a plurality of upper spacers 208 and a plurality of lower spacers 209, which are aligned in pairs in the Z direction of thickness of the collector 201 and designed to maintain a constant distance between the upper wall 202 and the lower wall 204 when the collector 201 is placed under vacuum. Each upper spacer 208 is positioned between the upper wall 202 and a portion 261 of the absorber panel 206 which is in thermal contact with the pipe 207, while each lower spacer 209 is positioned between the lower wall 204 and the pipe 207.

In this third embodiment, the upper spacers 208 are features in relief which project with respect to the upper wall 202, formed by rolling the sheet of glass of which the wall 202 is made, and which are advantageously toughened when the wall 202 is being heat toughened. The lower spacers 209 for their part are features in relief which project with respect to the lower wall 204, formed by embossing the metal wall 204. Advantageously, an insulating sheet 214 is added between the pipe 207 and the metal lower spacers 209 in order to reduce thermal losses. This sheet is preferably non-porous in order to make it easier to place the collector under vacuum and may, for example, be made of glass or of ceramics, with a thickness of the order of 1 to 4 millimeters. When such an insulating sheet is present in the collector 201 between the pipe 207 and the lower spacers 209, the lower spacers 209 are preferably aligned with the pipe 207 in the Z direction, although this is not compulsory.

Whatever the embodiment, each upper spacer 8, 108, 208 or lower spacer 9, 109, 209 has a thickness e₈, e₁₀₈, e₂₀₈ or e₉, e₁₀₉, e₂₀₉ less than 4 millimeters, preferably less than 2 millimeters. More specifically, when the spacers are glass beads that have undergone a chemical toughening treatment as in the second embodiment, these beads preferably have a thickness less than 2 millimeters. When the spacers are made of glass, of one piece with a glass wall of the collector and heat toughened during the heat toughening of the wall, as is the case with the rolled spacers 8, 9 and 208 of the first and third embodiments, these spacers preferably have a thickness less than 1 millimeter.

In addition, each spacer 8, 108, 208, 9, 109, 209 advantageously has a shape that is rounded in the direction of the absorption means, particularly a spherical or hemispherical shape, so as to minimize the area of contact between the spacer and the absorption means and thus limit thermal losses through the spacer. Attempts are also made at minimizing the spacers density on each of the upper and lower walls, notably by maximizing the value of the pitch p between the spacers, so as to limit the thermal losses through the spacers. More specifically, the value of the pitch p between the spacers on each wall is adjusted in order to reach a compromise between, on the one hand, minimizing the thermal losses through the spacers and, on the other hand, the distribution of stress through the wall. Increasing the pitch p between the spacers leads to an increase in the concentration of the mechanical stresses generated in the wall, and therefore increases the risk of this wall breaking when it is made of glass. A good compromise is obtained for a value of the pitch p between the spacers between 20 and 100 millimeters.

As is clear from the three embodiments described above, a solar collector according to the invention provides good transmission of solar radiation towards the absorption means of the collector by virtue both of the transparency of the upper wall and of the transparency of the upper spacers. In particular, by virtue of the transparency of the upper spacers, the entire active surface area of the absorber panel is exposed to the solar radiation, making it possible to improve the collection of energy by the absorber panel. For preference, the upper wall and/or the upper spacers are made of a transparent clear or extra-clear glass with a very low iron oxides content, such as the glass marketed by Saint-Gobain Glass in the “DIAMANT” range or, particularly when the upper spacers are features in relief obtained by rolling the upper wall of the collector, in the “ALBARINO” range manufactured by rolling.

A solar collector according to the invention also makes it possible to limit thermal losses by virtue, on the one hand, of the creation of the vacuum in the housing that accommodates the absorption means and, on the other hand, of the specific way in which the spacers are arranged in the collector. Positioning the spacers at the cold points of the absorption means, in the region of the pipe through which the heat-transfer fluid flows, effectively makes it possible to reduce the thermal losses through the spacers, as does the rounded shape of each spacer in the direction of the absorption means, which allows the area of contact between the spacer and the absorption means to be minimized. Strengthening the spacers, notably by heat or chemical toughening in the case of spacers made of glass, moreover makes it possible to reduce the number of spacers required to achieve the mechanical integrity of the collector, and this further contributes toward reducing the thermal losses through the spacers.

Because of the improved collection of energy by the absorber panel and because of the reduction in thermal losses, a solar collector according to the invention can have an energy conversion efficiency that exceeds the efficiencies of the solar collectors of the prior art.

A solar collector according to the invention may also have a very compact structure by virtue, on the one hand, of the use of walls and spacers made of toughened glass or of metal which, even they have relatively small thicknesses, exhibit good mechanical strength and, on the other hand, of the possibility of effectively maintaining a vacuum in the housing that accommodates the absorption means through the fitting of a glass-metal seal as described earlier. According to the invention, the thickness e₁, e₁₀₁, e₂₀₁ of the solar collector is less than 30 millimeters, preferably less than 25 millimeters. Thanks to its compactness, a solar collector according to the invention can easily and esthetically be incorporated into a roof or a façade of a building. The use of a sheet of toughened glass for the upper wall guarantees that the collector will be able to withstand bad weather when mounted on a roof or a façade of a building. Further, because it is evacuated, a solar collector according to the invention can improve the thermal insulation of the roof or the façade to which it is fitted.

Moreover, a solar collector according to the invention, when it comprises spacers formed collectively on one wall of the collector, eliminates the need to fit spacers individually in the collector. This is notably the case when the collector comprises rolled or embossed spacers as described earlier, or alternatively spacers formed by applying a glass frit to the wall by screen printing, in a single pass of the squeegee, then by baking this glass frit. Thanks to this collective manufacturing of the spacers, the method for manufacturing the solar collector is simpler and faster, which is advantageous for manufacture on an industrial scale.

A solar collector according to the invention advantageously forms part of a cladding assembly 20, 320 intended to be mounted on a structure, such as a roof or a façade of a building, in which the cladding assembly 20, 320 comprises other elements, notably other solar collectors and/or photovoltaic modules and/or conventional tiles or slates. The various elements of the cladding assembly 20, 320 are preferably arranged in relation to one another in a stepped arrangement with overlap, in the manner of tiles or slates, and joined together by fixing means which have not been depicted, for example hooks and rails with saw-tooth edges as described in US-A-2003/0213201. As illustrated in the fourth embodiment visible in FIG. 5, the shape of solar collectors according to the invention can easily be adapted to facilitate a stepped arrangement of the collectors on a roof or façade of a building, with overlap in the manner of tiles or slates.

In this fourth embodiment, the elements similar to those of the first embodiment bear identical references increased by 300. Each solar collector 301 according to this fourth embodiment has the same structure as any one of the collectors 1, 101, 201 described earlier, except that the upper wall 302 of the collector 301 is given a height h₃₀₂, in a height direction Y of the collector 301, that exceeds the height in the Y direction of the underlying parts of the collector, and is notably greater than the height h₃₀₄ of the lower wall 304 of the collector. In addition, when the collector 301 is in the assembled configuration, the walls 302 and 304 are arranged in such a way that two opposite free edges 321 and 323 of the upper wall 302 protrude a distance d beyond the corresponding free edges 341 and 343 of the lower wall 304.

When the structure that accommodates the collector 301 is inclined with respect to the horizontal, as is the case with the structure 312 depicted in FIG. 5, the collector 301 is intended to be mounted on the structure with its height direction Y substantially aligned with the direction of inclination of the structure with respect to the horizontal. Thus, when the collector 301 is in the configuration in which it is mounted on the inclined structure 312, the upper wall 302 of the collector 301 is able partially to cover the upper wall 302 of an analogous second collector 301 mounted on the structure 312 juxtaposed below the first collector 301 in the direction of inclination of the structure. This then yields a cladding assembly 320 comprising several collectors 301 with the desired stepped arrangement, in the manner of tiles, on the structure. Such a stepped arrangement of the collectors 301 gives the cladding assembly 320 good weather tightness, even when faced with strong wind.

The invention is not restricted to the examples described and depicted. In particular:

-   the upper and lower spacers could be not of one piece (or integral)     with the walls of the solar collector but added on to these walls by     any appropriate means, notably by bonding or welding; -   the upper spacers may be made of any transparent material other than     glass and capable of withstanding high compressive mechanical     stresses, for example of plastic; -   the lower spacers may be made of any material, transparent or     otherwise, able to withstand high mechanical stresses in     compression, for example glass, metal, ceramics; -   glass spacers incorporated into a glass wall of the solar collector     may be obtained by techniques other than rolling, notably by     applying a glass frit to the wall, either by screen printing, which     means that all the spacers can be applied in a single pass of the     squeegee, or by a dispenser, making it possible to apply relatively     thick spacers, the baking of the frit in any event advantageously     occurring during the heat toughening of the wall; as in the case of     the rolled spacers, these glass spacers of one piece with a glass     wall preferably have a thickness less than 1 millimeter; -   when the upper spacers are obtained by rolling the upper glass wall     of the solar collector, that face of the upper wall that is intended     to face toward the outside of the collector may also be rolled so     that it has a non-slip texture; that is particularly advantageous     when the solar collector is installed on the roof of a building, to     prevent anybody moving around on the roof from falling off; -   the face of the upper wall that is intended to face toward the     outside of the collector may also be rolled to give it a texture of     the lenticular type, so that an image can be viewed on the surface     of the solar collector without this obstructing the direct rays of     the sun, as described in FR-A-2 896 596; -   the upper and lower spacers may be of any shape suited to their     function, preferably rounded in the direction of the absorption     means; the upper and lower spacers may notably be in the form of     pads of circular or polygonal cross section as described earlier, or     alternatively in the form of elongate segments the longitudinal     direction of which preferably runs transversally to the longitudinal     direction of the or each pipe in which the heat-transfer fluid     flows; in the case of a collector comprising a lower wall made of     metal, this wall may be corrugated to form projecting segments which     act as lower spacers; -   to improve the mechanical strength of the solar collector, the upper     and/or lower wall of the collector, when made of glass, may be     assembled with another sheet of glass via a polymer interlayer to     form a sheet of laminated glass; -   in order to protect the seal between a glass wall and the metal tray     or frame of the collector against thermal stresses liable to cause     damage to the seal and therefore to the sealing of the collector,     notably in the vicinity of the connectors installed in the metal     tray or frame, the metal tray or frame may, so that it can be     connected to the glass wall, comprise a thinner wall offset toward     the outside in relation to the seal, beyond the edges of the glass     wall, so as to create an insulating empty space in line with the     region of the glass-metal join; -   the collector may comprise several pipes in which the heat-transfer     fluid flows, rather than a single pipe in the form of a serpentine     coil; -   the or each pipe for the circulation of heat-transfer fluid may,     rather than being a pipe independent of the absorber panel as shown     in FIGS. 1 to 3, be formed at least in part by the absorber panel,     for example by securing a U-shaped component to the lower face of     the absorber panel; -   the absorption means arranged in the housing of the solar collector     may, in addition to comprising means of converting the energy     originating from solar radiation into thermal energy, such as an     absorber panel and a pipe through which a heat-transfer fluid flows,     as described earlier, comprise means for converting the energy     originating from solar radiation into electrical energy, such as     photovoltaic cells. 

1. A solar collector comprising: a first wall and a second wall facing one another, which between them delimit a housing, the first wall being transparent and intended to face in a direction of a solar radiation incident upon the collector, an absorption device configured to absorb the energy originating from the solar radiation and positioned in the housing the absorption device comprising at least one pipe through which a heat-transfer fluid flows, and at least one transparent spacer arranged between the first wall and the absorption device in a region of the pipe.
 2. The solar collector as claimed in claim 1, wherein the absorption device comprises an absorbing panel thermally connected to the pipe, the spacer being arranged between the first wall and a portion of the absorber panel in thermal contact with the pipe.
 3. The solar collector as claimed in claim 1, wherein the spacer has a shape that is rounded in the direction of the absorption device.
 4. The solar collector as claimed in claim 1, wherein the spacer is of one piece with the first wall.
 5. The solar collector as claimed in claim 4, wherein the first wall and the spacer are made of toughened glass, the spacer being heat toughened with the first wall.
 6. The solar collector as claimed in claim 1, wherein the first wall is made of glass, the spacer being a feature in relief with respect to the first wall obtained by rolling the first wall.
 7. The solar collector as claimed in claim 1, wherein the spacer is a spacer made of chemically toughened glass.
 8. The solar collector as claimed in claim 1, wherein the spacer has a thickness less than 4 millimeters.
 9. The solar collector as claimed in claim 1, comprising at least one pair of spacers arranged one on each side of the pipe, comprising the first spacer between the first wall and the absorption device and a second spacer between the second wall and the absorption device.
 10. The solar collector as claimed in claim 9, wherein the second wall is made of metal, the second spacer being a feature in relief with respect to the second wall obtained by embossing the second wall.
 11. The solar collector as claimed in claim 9, wherein the second wall is identical to the first wall and symmetric with the latter with respect to a median plane of the collector.
 12. The solar collector as claimed in claim 1, wherein the housing is fluidtight and configured to be placed under vacuum.
 13. The solar collector as claimed in claim 1, comprising a spacer frame between the first and second walls.
 14. The solar collector as claimed in claim 1, wherein the solar collector has a thickness less than 30 millimeters.
 15. A cladding assembly for a structure, particularly a roof or a façade of a building, comprising at least one solar collector as claimed in claim
 1. 